1. Field of the Invention
The present invention relates generally to a new interconnection and a method for making and detaching the same, and more particularly, to a solder connection. This invention addresses the card assembly/rework issues associated with column and ball grid array (CGA and BGA) structures by introducing an interconnection structure with a transient liquid solder interface that includes fine metal powder additives.
2. Description of Related Art
Ceramic column grid array (CGA) packages are being used for many high performance ASIC chips and microprocessor chips for IBM and original engineering manufacturer (OEM) customers. The preferred processes for lowest cost manufacturing and minimum handling damage during bond, assembly, and test are the wire CGA process and the ball grid array (BGA) process. However, inherent to the joining process of the wire CGA and BGA structure is an eutectic interface used in the column or ball attachment to the substrate pads. As a result, during circuit card joining and reworking of these packages, two problems are typically experienced. First, during joining, the solder composition attaching the columns to the substrate can melt causing the columns to tilt, thus, putting the interconnect structure out of alignment. Second, during rework to remove the CGA or BGA, columns or balls may be left on the circuit card. Currently, since the columns and balls are made of 90/10 Pb/Sn solder, this eutectic material residue remaining on the chip terminals and/or testing substrate pads must be individually removed. This is a labor intensive process that also critically increases the length of time that the card site must be kept within the temperature range necessary to facilitate removal of all the columns or balls (200-220.degree. C. ). Typically, a hand held hot gas tool with an integral vacuum nozzle is moved back and forth over the card site. The hot gas melts the eutectic solder on the circuit card lands, and the vacuum picks up the balls or columns from the lands. This process may be repeated, especially with columns which tilt or detach from the substrate during the vacuum "sweep" operation. Repeated localized heating to pick up columns has led to circuit card land delaminations causing the entire card to be unusable. Additionally, this problem will further develop as package sizes increase, and more columns or balls are added to the structure.
The prior art has dealt with these deficiencies in a number of diverse ways. In U.S. Pat. No. 5,130,779, issued to Agarwala et al., on Jul. 14, 1992, entitled, "SOLDER MASS HAVING CONDUCTIVE ENCAPSULATING ARRANGEMENT", an elongated solder interconnection is formed on an electronic carrier by capping a metal layer on a deposited solder mass. A further elongated solder interconnection is then formed by the addition of a second solder mass on the first solder mass capped with the metal layer. The melting point of the subsequent solder mass may be different than the first solder mass to facilitate component removal. The different melting points allow the low-melt solder to reflow to its mating substrate pad at a temperature lower than the high-melt solder's melting point. The primary purpose of the metal barrier layer is to prevent the migration of the intermediate melting point solder towards the low melt solder. Thus, while the low-melt solder flows, the high-melt solder remains in the solid state, as it does prior to the reflow cycle. Upon reflow, the encapsulated, high-melt solder on the electronic components and the low-melt solder deposited over this metal barrier layer forms a new solder mass having an intermediate melting point, which is higher than the melting point of the low-melt solder mass on the other side of the barrier layer. Thus, the electronic components can later be separated, as the low-melt solder mass would melt prior to the intermediate melting point solder mass. This prior art patent, however, does not address creating a transient liquid solder interface with fine metal powder additives, which after reflow and attachment to the substrate pads create a higher effective melting point interface.
In U.S. Pat. No. 5,326,016, issued to Cohen et al. on Jul. 5, 1994, entitled, "METHOD FOR REMOVING ELECTRICAL COMPONENTS FROM PRINTED CIRCUIT BOARDS", the leads of each component are connected to circuitry on the circuit board by a connection alloy comprised of two constituent metals and having a given melting point less than that of either of the constituent metals. A removal alloy, having a particular melting point substantially below the given melting point of the connection alloy, and typically in the form of a wire, is heated to a temperature greater than its particular melting point but below the given melting point of the connection alloy. Next, the connection alloy on each of the leads of the integrated circuit are contacted by the molten removal alloy causing a reaction that also produces a molten state for the connection alloy. After the connection alloy has reached a molten state, the integrated circuit is safely separated from the circuit board. This process, however, does not utilize a metal powder mixed in the interface solder, balls and columns, being joined to the substrate prior to any card attachment or rework. Also, this process lowers the melting point of the interconnection joint to allow removal. In contrast, metal powder additions effectively increase the melting point of the interface, instead of lowering it. Metal powder additions do not melt during ball or column attachment to the substrate, or during card assembly or rework.
In U.S. Pat. No. 5,234,149, issued to Katz et al. on Aug. 10, 1993, entitled, "DEBONDABLE METALLIC BONDING METHOD", a method of joining a device or carrier to another substrate is taught, providing a bondable first metallurgy on the device side for the solder connections (ball) and providing a second metallurgy on the substrate side to which these solder balls can make electrical contact under pressure, but not wet, to create a permanent metallurgical bond. The solder ball is heated to a temperature that facilitates wetting the first but not the second metallization, whereby the device can be mechanically pulled away from the substrate while the melted solder balls are immersed in the liquid flux. However, there is no disclosure regarding changing the interface solder melting point by addition of high melting reacting powders.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an interconnection structure between a device substrate or module assembly and a circuit card.
It is another object of the present invention to provide a low melting point solder layer that allows for an integrated circuit chip to be attached to the next level of packaging at substantially lower temperatures (approximately 100.degree. C. less) than would the high melting point solder.
A further object of the present invention is to provide for a number of variations for a transient melting solder comprising a low melting alloy powder/powder mixture and a higher melting alloy additive in the form of a powder.
Another object of the present invention is to minimize the likelihood of handling damage occurring on the relatively fragile interconnect columns.
Still other objects of the invention will in part be obvious and will in part be apparent from the specification.